1. Field of the Invention
The present invention relates to a method of molding powders of metal, ceramic and the like into compression moldings of complicated shapes.
2. Description of the Prior Art
Various methods of producing machine parts of high density and intricate shapes from powders of metals and ceramics by the combination of injection molding and sintering techniques are well known.
For example, the Wiech process comprises kneading metal powder of about 10 to 15 .mu.m and a thermoplastic resin and preparing pellets, injection molding the pellets by the use of an oversized mold in consideration of the desired shrinkage allowance, degreasing the resulting molding by the application of heat or by solvent extraction to make it porous and then densifying the porous molding by a sintering operation and this process is used for the production of intricately shaped machine parts from iron nickel alloy, stainless steel, etc.
Also known in the art are techniques for the injection molding of sintered hard alloy, stellite, tool steel, superalloy, titanium, etc., and techniques for the injection molding of alumina, zirconia, silicon nitride, silicon carbide, sialon (Si-Al-O-N), graphite short fiber, etc.
More specifically, techniques are known for the manufacture for example of turbocharger rotors for automobile engines, turbine rotors for gas turbine engines, etc., by the injection molding of silicon nitride and silicon carbide.
While the injection molding methods used widely with these techniques have the advantage of ensuring high dimensional accuracy for products, they also have some disadvantages as enumerated below.
(1) Since a binder of as much as 30 to 40 volume % is added to provide a powder material with plasticity, a considerably long time is required for the degreasing operation and this does not conform with the injection molding techniques which should essentially be suited for the purpose of mass production in short time thus failing to enjoy the intended economic effect.
(2) Since the injection molds are expensive, the injection molding methods are not suited for multikind and small quantity production purposes.
(3) It is difficult to mold thick-walled parts without internal defects.
(4) Sophisticated technological accumulation as to the additon of binders and the selection of injection molding conditions is necessary and the occurrence of voids within moldings or the occurrence of flow marks on moldings will be caused if these conditions are improper.
In addition to these methods, there is another method of this kind of techniques in which after a powder material has been packed in a mold, the powder material is molded under the application of a hydrostatic pressure of about 2000 to 4000 atm (2026.5.times.10.sup.5 to 4053.times.10.sup.5 Pa) by the cold isostatic press (CIP) process employing water or oil and then the material is transferred to a sintering stage thereby obtaining the final product.
With this method employing the CIP process, the hydraulic pressure is uniformly applied to a material to be molded and thus under the ideal conditions the density of a molding becomes uniform making it possible to mold parts of complicate shapes. Its first feature is the use of an inexpensive rubber mold and its second feature is the nonuse of any binder or the use of a very small amount of binder in the case of a granular powder material thus eliminating the disadvantage of the above (1). Also, its third feature resides in that the method is applicable to the production of thick-walled parts and this fact makes it possible to enjoy the advantage of not being subjected to the limitations due to the degreasing. Its fourth feature is the fact that there is no need for such sophisticated technological accumulation as in the case of the injection molding machine and its fifth feature resides in that although the mass processing in such a short period of times the injection molding is not possible, the elimination of the degreasing operation ensures, when considered in the light of the CIP process on the whole, a high degree of freedom which allows its use in applications ranging from the scant kind and mass production to the multikind and small quantity production.
The CIP processes are roughly divided into two types one of which is a wet-bag type and the other is a dry-bag type and here the subject interest is the wet-bag type which is suited for the molding of parts of complicated shapes due to the reduced limitations to the shape of the rubber mold.
With the CIP process having a number of advantages as mentioned above, however, the most serious disadvantage is inferiority in the dimensional accuracy of moldings (the accuracy is said to be in the range of .+-.0.3 and 1.5% at the most) and therefore the CIP process cannot be used for the production of parts requiring a high degree of dimensional accuracy.
In this respect, Japanese Patent Publication No. 37383/1972 discloses a method comprising inserting a rubber bag into a mold of a given shape, packing a powder material in the rubber bag, reducing the pressure within the bag and removing the rubber bag packed with the powder material from the mold while maintaining the shape of the mold and then subjecting the bag as such to the molding operation by an isostatic press and in this method the procedure of inserting into the mold a thin rubber bag conforming with its inside involves difficulty thus making it difficult for this method to produce moldings having a high degree of dimensional accuracy.
As mentioned hereinabove, the conventional methods have their own merits and demerits so that even any one of these methods is used, it is difficult to perform the CIP process if the merits and demerits of the method do not conform well with products to be molded.